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class UnboundedStack<T> {
ghost var representation: set<object>;
ghost var content: seq<T>;
var top: Node<T>;
function IsUnboundedStack(): bool
reads this, representation;
{
this in representation &&
(top == null ==>
content == []) &&
(top != null ==>
top in representation && this !in top.footprint && top.footprint <= representation &&
content == top.content &&
top.Valid())
}
method InitUnboundedStack()
modifies this;
ensures IsUnboundedStack();
ensures content == [];
{
this.top := null;
this.content := [];
this.representation := {this};
}
method Push(val: T)
requires IsUnboundedStack();
modifies this;
ensures IsUnboundedStack();
ensures content == [val] + old(content);
{
top := new Node<T>.InitNode(val,top);
representation := representation + top.footprint;
content := [val] + content;
}
method Pop() returns (result: T)
requires IsUnboundedStack();
requires content != [];
modifies this;
ensures IsUnboundedStack();
ensures content == old(content)[1..];
{
result := top.val;
top := top.next;
content := content[1..];
}
method isEmpty() returns (result: bool)
requires IsUnboundedStack();
ensures result <==> content == [];
{
result := top == null;
}
}
class Node<T> {
ghost var footprint: set<object>;
ghost var content: seq<T>;
var val: T;
var next: Node<T>;
function Valid(): bool
reads this, footprint;
{
this in footprint &&
(next == null ==>
content == [val]) &&
(next != null ==>
next in footprint && next.footprint <= footprint && this !in next.footprint &&
content == [val] + next.content &&
next.Valid())
}
method InitNode(val: T, next: Node<T>)
requires next != null ==> next.Valid() && !(this in next.footprint);
modifies this;
ensures Valid();
ensures next != null ==> content == [val] + next.content &&
footprint == {this} + next.footprint;
ensures next == null ==> content == [val] &&
footprint == {this};
{
this.val := val;
this.next := next;
if (next == null) {
this.footprint := {this};
this.content := [val];
} else {
this.footprint := {this} + next.footprint;
this.content := [val] + next.content;
}
}
}
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